(a) Field of the Technology
This invention relates to a single crystal of compound semiconductor of groups III-V with low dislocation density.
The elements of group III on the periodic table are B, Al, Ga, In, etc. The elements of group V are N, P, As, Sb, etc. Semiconductors consisting of these elements are GaAs, InSb, GaP, GaSb and other compounds.
Compound semiconductors of groups III-V are used as substrates for field effect transistors, integrated circuits including them, light emitting devices, light detecting devices or various optical integrated circuits.
Dislocation density signifies the numbers of dislocation of lattices in a single crystal in a unit volume. Dislocation density is frequently represented by EPD (Etch Pit Density). A single crystal is sliced into thin wafers. A wafer is then etched with a pertinent etchant which reveals superficial dislocations as etch pits. EPD is defined as a number of etch pits per unit area. An operator counts the number of etch pits in a definite area on a wafer through a microscope and calculates EPD by dividing the number by the area.
Although EPD is one of the practical representations of dislocation density, the terms EPD and dislocation density have the same meaning in this specification.
It is desirable that dislocation density is low, and the distribution of dislocation density is uniform in a wafer. It may be best, that the distribution of dislocation density is uniform throughout a single crystal ingot.
However, for instance, GaAs single crystals grown by conventional LEC method (Liquid Encapsulated Czockralski Method) have a great many dislocations. EPD is usually 50,000 to 100,000 cm.sup.-2.
The LEC method is one of the pulling methods for crystal growth. A single crystal is pulled up into B.sub.2 O.sub.3 from a material melt. The temperature gradient in the crystal is very large near the solid-liquid interface. Big thermal stress is generated and great thermal distortions occur because of this stress. Prevailing thermal distortions multiply dislocations in the single crystal.
The HB method (Horizontal Bridgman Method) is one of the boat methods for crystal growth. It uses a boat containing material melt and grows a single crystal by changing the temperature distribution in a horizontal direction. This method can make a single crystal with a low EPD, since it has so high a degree of freedom for controlling temperature distributions. The generation of thermal stress is reduced.
However a single crystal made by the HB method does not have a round section. The section resembles the character "D". Large parts of a crystal are wasted in a grinding process to make round wafers.
Besides, because the HB method uses a quartz boat, the grown crystal is apt to include silicon. Therefore the crystals grown by the HB method have low resistivity in general. To obtain semi-insulation which is an indispensable characteristic for FET substrates, we must dope Cr, etc. into a crystal.
This invention has a wide applicability for any methods for growing a single crystal of compound semiconductors. Namely it is fully applicable to LEC method, HB method and other methods.